The present invention relates to axial piston hydraulic pumps and motors and, in particular, to improved charge-discharge valving between the main fluid inlet-outlet couplings and the cylinders.
In a widely used standard design of axial-piston hydraulic pump-motors, the cylinder barrel is keyed to a main shaft which extends the full length of the machine and is journaled in both ends of the housing. A swash plate is connected by a universal joint to the shaft, is suitably coupled to the piston rods, such as by balls and sockets, and bears against an adjustable thrust and radial bearing assembly.
Fluid is charged to and discharged from the cylinders through ports in the outer end of the barrel which sweep across semi-annular slots in a valve plate on the end wall of the housing. The design of the cylinder ports and the valve plate is critical; the design objective is to maintain a fluid pressure on the land areas of the valve plate and barrel which will approximately balance a pressure exerted on the ends of the cylinder bores in the direction toward the valve plate, thus to ensure maintenance of a pressurized fluid film between the rotating barrel and the fixed valve plate throughout the range of working pressures of the machine while minimizing leakage at the valve plate. This design principle is reasonably effective when properly executed in respect of both the engineering and precision manufacturing of the machine, but excellence is difficult to attain and the failure to achieve it can greatly reduce machine efficiency and increase machine failure rates. Moreover, the conventional valving design inherently requires a reduction in cross-sectional area along the path between the main fluid inlet and outlet ports, specifically at the ends of the cylinder bores, to generate a hydraulic force on the barrel acting toward the valve plate. (Note that the barrel is movable axially on the main shaft). The constriction in the flow path to and from each cylinder produces turbulence, energy-consuming pressure changes and, under some conditions, especially on the intake stroke in the pump mode, cavitation. Turbulence is an energy loss, or more accurately, an energy exchange almost always lost as heat that must be removed from the system. The pressure changes at the constrictions in the flow paths reduce volumetric efficiency. Cavitation produces harmful cavitation corrosion, thus reducing machine life, increases noise, and limits operating speed and, therefore, horsepower for a given displacement.
The design of conventional ports and valve plates is largely impirical and imprecise; the effects of friction, displacement and speed adjustments and changes in operation, a change from motor to pump mode, scale-up or scale-down, and many other factors affect the design parameters and make perfection of a given design costly. The hydraulic force on the barrel that maintains the pressure balance seal at the valve plate is inherently unbalanced (assymetrical with respect to the barrel axis) and provides a non-uniform sealing effect due to the pressure differences at the barrel-plate interface. Practical operation outside fairly tight operating specifications can significantly reduce efficiency or result in breakdown.